In a magnetic resonance imaging system (abbreviated “magnetic resonance system”) the body to be examined may be exposed to a relatively high main magnetic field (the so-called “B0 field”), for example of 1.5, 3, or 7 Tesla, with the aid of a main field magnet system. Additionally, a magnetic field gradient is applied with the aid of a gradient system. Using a radiofrequency transmission system, radiofrequency excitation signals (RF signals) are emitted by suitable antenna apparatuses. The RF signals are intended to lead to the nuclear spin of specific atoms or molecules that are excited resonantly by this radiofrequency field, being tilted by a defined flip angle in relation to the magnetic field lines of the main magnetic field. This radiofrequency excitation or the resulting flip angle distribution will also be referred to as nuclear magnetization or, briefly, “magnetization” in the following text. During the relaxation of the nuclear spins, radiofrequency signals, so-called magnetic resonance signals, are emitted, and are received by suitable reception antennas and processed further. From the raw data acquired thus, it is possible to reconstruct the desired image data. The radiofrequency signals (the so-called “B1 field”) for nuclear spin magnetization may be emitted by a so-called “whole body coil” that is arranged securely in the device about the measurement space (patient tunnel). Magnetic resonance signals may be received with the aid of so-called local coils that are positioned more closely on the body of the patient. However, in principle, magnetic resonance signals may also be received by the whole body coil and/or the RF signals may be transmitted using local coils.
For a specific measurement, a magnetic resonance system actuation sequence (also abbreviated as “actuation sequence” in the following text) with a radiofrequency pulse train (RF pulse train) to be emitted and, to be applied coordinated therewith, a gradient pulse train (with matching gradient pulses in the slice selection direction, in the phase encoding direction and in the readout direction, often the z-direction, y-direction and z-direction) are generated in advance. Further control prescriptions are generated in advance, wherein a multiplicity of control prescriptions, like the parameters for the actuation sequence, are defined in a so-called measurement protocol or control protocol. By way of example, this measurement protocol may be recalled from a memory for a specific measurement and may be modified in situ by the user. During the measurement, the magnetic resonance system is controlled completely automatically on the basis of this actuation sequence, wherein the control apparatus of the magnetic resonance system reads out commands from the measurement protocol and works through the commands.
In order to generate the actuation sequences of, in particular, an RF pulse train, a target magnetization, (e.g., a desired spatial flip angle distribution), may be predetermined (by the measurement protocol and/or by the user). Using a suitable RF pulse optimization program that may operate using a numerical optimization method using a target function to be minimized, the matching RF pulse sequence is calculated such that this target magnetization is reached.
To this end, current “field distribution maps”, e.g., field distribution maps determined with the current examination object and the current examination arrangement, may be used. These field distribution maps include the “B1 maps” that each specify the spatial B1 field distribution for a specific transmission antenna element, e.g., the spatial sensitivity of the transmission antenna element, and the “B0 maps”, which represent, in a spatially resolved manner, the off resonances or deviations of the B0 field from the actually desired homogenous B0 field (e.g., the actually sought-after Larmor frequency). These field distribution maps are taken into account in the optimization method in order to find the ideal actuation sequence for the measurement to be carried out for the current examination object in the current examination environment.
In so doing, the information from the B1 maps and B0 maps is used in the target function in order to be able to take into account inhomogeneities of the B1 field or geometric distortions, e.g., due to radiofrequency shimming in the case of a spatially selective excitation by the transmission antenna elements, etc., and inhomogeneities of the B0 field in order to eliminate, or at least greatly reduce, falsification of the raw data for the magnetic resonance images caused thereby. In the case of so-called parallel transmission methods (pTX methods), radiofrequency pulses are emitted by several independent transmission channels or transmission antenna elements in order to be superposed in the measurement space for achieving an individually definable radiofrequency field. Knowledge about the spatial sensitivity of the transmission coils in question and the present off resonance of the B0 field in respect of the current examination object is an important requirement for being able to calculate suitable pTX-RF pulse sequences.
On the other hand, the amount of data in the field distribution maps, which are included in the numerical optimization process, also has a significant influence on the computational complexity of the optimization method. If the data relate to multi-slice applications, such as in e.g., fMRI (functional magnetic resonance imaging) methods, DWI (diffusion weighted imaging) methods and DTI (diffusion tensor imaging) methods, the data load within the optimization method caused by the field distribution maps becomes problematic, especially in view of the following aspects.
The acquisition of the field distribution maps, e.g., the B1 maps and the B0 maps, is relatively time-consuming for a multiplicity of slices and substantially increases the overall examination duration within the clinical routine. This problem becomes more pronounced if a dynamic update, e.g., a reacquisition, of such field distribution maps is necessary due to patient movements.
The calculation time required to calculate a pTX-RF pulse sequence, specially adapted to the examination object or the examination situation, per slice, becomes unacceptable. Currently, the calculation for a single slice is already a challenge. This applies ever more so if specific restrictions of the hardware. For example, restrictions of the radiofrequency amplifiers or of the gradient system, and the specific absorption rate restrictions (SAR restriction) also have to be taken into account during the pulse sequence calculation.
A theoretically possible solution may lie in calculating a common, identical pTX radiofrequency pulse sequence for all slices to be excited, which is successively applied to the slices, rather than calculating an individual pulse sequence for each individual slice. This offers at least a compromise solution, but is in no way ideal. Although, to this end, several slices may be linked to one another in order to combine the individual optimization for the individual slices to form a single, but large optimization problem, the optimization problem to be solved numerically rapidly grows to huge dimensions, and the dimensions exceed the calculation capacities of standard CPUs and standard RAM hardware. Therefore, specific hardware-technical solutions may be required for this, which may make the devices significantly more expensive.